The invention relates to a method for detecting the distance between an object and an ultrasonic objective which can be moved relative to each other, and to an arrangement for carrying out the method.
U.S. Pat. No. 4,356,392 discloses an optical imaging system having an opto-electronic detection arrangement for determining a deviation between the image plane of the imaging system and a second plane in which an image is produced. The detection arrangement has a radiation source which generates an auxiliary beam and a reflection element in the radiation path of the auxiliary beam which is reflected a first time and redirected to the second plane by means of the reflection element. Two detectors in the radiation path of the auxiliary radiation beam, which is reflected twice at the second plane, are connected to the imaging system, and the difference between the output signals of the two detectors represents a measure of the deviation in distance. This familiar arrangement requires transmission elements, which are coupled to the imaging system, in the form of two additional lens systems and reflection elements and is not intended, nor is it suitable, for detecting the distance between an object and an ultrasonic objective which can be moved relative to each other.
When working with an acoustical microscope, in which working distances are of the order of magnitude of about 30 microns, reproducible focussing of the ultrasonic objective is possible to an accuracy of better than 0.1 microns by mechanically adjusting the microscope gauge as disclosed in Leitz-Mitt. Wiss. u. Techn., Vol. VIII, No. 3/4, p.65, Wetzlar, May 1982. The arrangement shown in this publication consists of an acoustical microscope and an optical microscope which are mounted on a common support and are aligned with each other in such a manner that object details selected or identified with the optical microscope can be retrieved with only very little deviation in the acoustical microscope image. The actual final focussing on the object surface for generating the ultrasonic image is carried out by observing the contrast of the acoustical image which changes considerably within the focussing range even with a change of approximately 0.1-0.2 microns, that is, less than one quarter of the wavelength of sound in the water coupling medium. Since the front face of the lens body of the ultrasonic objective is then moved in a raster scanning pattern over the object surface at the above-mentioned working distance in two directions which are perpendicular to each other, any contact with the object will lead to contamination or even damage of the ultrasonic objective and/or of the object and thus to an operational failure of the device.
Since examinations with the acoustical microscope are also frequently carried out in a darkened room, the risks mentioned above are also incurred to the same extent even during adjustment work.